1. Technical Field
The present disclosure relates to thermoacoustic devices, particularly, to a carbon nanotube based thermoacoustic device with a heating dissipating structure.
2. Description of Related Art
A typical speaker is an electro-acoustic transducer that converts electrical signals into sound. Different types of speakers can be categorized according to their working principles, such as electro-dynamic speakers, electromagnetic speakers, electrostatic speakers and piezoelectric speakers. However, these types use mechanical vibration to produce sound waves by “electro-mechanical-acoustic” conversion. Among the various types, the electro-dynamic speakers are most widely used.
Referring to FIG. 12, the electro-dynamic speaker 500 typically includes a voice coil 502, a magnet 504 and a cone 506. The voice coil 502 is an electrical conductor, and is placed in the magnetic field of the magnet 504. By applying an electrical current to the voice coil 502, a mechanical vibration of the cone 506 is produced due to the interaction between the electromagnetic field produced by the voice coil 502 and the magnetic field of the magnets 504, thus producing sound waves by kinetically pushing the air. The structure of the electric-powered loudspeaker 500 is dependent on magnetic fields and often weighty magnets.
Thermoacoustic effect is the conversion of heat to acoustic signals. When signals are inputted into a thermoacoustic element, heating is produced in the thermoacoustic element according to the variations of the signal and/or signal strength. Heat is propagated into the surrounding medium. The heating of the medium causes thermal expansion and produces pressure waves in the surrounding medium, resulting in sound wave generation. Such an acoustic effect induced by temperature waves is commonly called “the thermoacoustic effect”.
A thermophone based on the thermoacoustic effect was created by H. D. Arnold and I. B. Crandall (H. D. Arnold and I. B. Crandall, “The thermophone as a precision source of sound”, Phys. Rev. 10, pp 22-38 (1917)). A platinum strip with a thickness of 7×10−5 cm was used as a thermoacoustic element. The heat capacity per unit area of the platinum strip with the thickness of 7×10−5 cm is 2×10−4 J/cm2*K. However, the thermophone adopting the platinum strip produces extremely weak sound.
Carbon nanotubes (CNT) are a novel carbonaceous material having extremely small size and extremely large specific surface area. Carbon nanotubes have received a great deal of interest since the early 1990s, and have interesting and potentially useful electrical and mechanical properties, and have been widely used in a plurality of fields. Fan et al. discloses a thermoacoustic device with simpler structure and smaller size, working without the magnet in an article of “Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers”, Fan et al., Nano Letters, Vol. 8 (12), 4539-4545 (2008). The thermoacoustic device includes a sound wave generator which is a carbon nanotube film. The carbon nanotube film used in the thermoacoustic device has a large specific surface area, and extremely small heat capacity per unit area. The sound wave generator emits sound with a wide frequency response range. Accordingly, the thermoacoustic device adopting the carbon nanotube film has a potential to be used in places of the loudspeakers of the prior art.
The carbon nanotube film is soft and can be easily damaged, thus, a base or support is usually adopted to support and protect the carbon nanotube film. However, during operation, the carbon nanotube film will eventually generate heat stored in the base, which may scald a user's hand or may burn anything near the base. The performance of the thermoacoustic device will be adversely affected.